My thesis: Ideas stick in your craw when they "fit" a certain way in the kinks of your mind - and what this means is that "sticky" ideas tend to be malleable and flexible and customizable (but not so vague they swoosh around like vapor instead of taking hold). Think of those gummy sticky balls that you get for a quarter from vending machines - the kind you're supposed to throw against a wall. Something like that.
Greg says taking ownership of something makes it sticky - it's got to be "mine" before I'll remember it or care. That means I've got to be able to smoosh the idea into something that's attractive to me - and that this smooshing needs to be insanely easy for something to be "sticky."
Erik makes lists and collections - his way of mashing an ontology to fit his mind. Alyshia brings up the idea that people around us (and what's sticking to them) are a compelling factor in what makes ideas stick to us - something that cross-platform (cross-brain) compatible has to be kludged to fit the varied people it transforms.
I'm severely overusing the word "stick" here.
People learn, in part, by making connections - and the more connections they can make between their old worldview and your new idea, the better chance you'll have of making your idea stay in their mind and carry on to action after you leave. Classic example: look at widespread world religions and you'll see how Christianity, Buddhism, etc. adapted by smooshing into local cultures? (Heck, the very timing of Christmas itself is an adaptation.) Other examples are food (McRiceburgers, Thai pizza), language (creoles and foreign loanwords) and PBL/Do-Learn (start with something concrete, then move into the abstract).
Sticky ideas are hackable. They have to be. The more ways you give people to work with, use, and sometimes transform your idea, the more they'll pick it up. An idea will never be sticky in exactly the way/shape/form you first have it in your brain - if you want it to get out and get spread by others, you must, to some extent, just let it go.